Look for green onions and scallions with bright-green, crisp tops and firm white bases. Select those with the most white up the stem. Refrigerated, wrapped in a plactic bag, for up to 5 days.
Most of this vegetable is edible. Slice off the white roots at the base and then slice as desired. Many recipes will specify white part only, or white and 3 inches of green. Scallions and green onions are good in salads, and they can be braised like baby leeks in butter and stock.